The present document is based on Japanese Priority Document JP 2001-186045, filed in the Japanese Patent Office on Jun. 20, 2001, the entire contents of which being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method for magnetic characteristics modulation and a magnetically functioning apparatus. More particularly, the present invention is concerned with a magnetic characteristics modulation method for changing the characteristics of a magnetic thin film by changing an electric field generated on the surface of the magnetic thin film, and a magnetically functioning apparatus.
2. Description of the Related Art
As an example of a magnetically functioning apparatus utilized in large scale integrated circuit (LSI), a magnetic random access memory (hereinafter, referred to simply as xe2x80x9cMRAMxe2x80x9d) 101 diagrammatically shown in FIG. 30 has been known An MRAM is constituted by a bit line 111, a word line 121 disposed in a direction perpendicular to the bit line 111, and a magnetic memory device 131 provided between the bit line 111 and the word line 121 which cross to each other, and a tunnel magnetoresistance device is used in the magnetic memory device 131.
In the further scaled-down devices, the MRAM 101 suffers inversion of magnetization and therefore, the strength of a magnetic field generated by a current is reduced, that is, the strength of the magnetic field obtained is reduced substantially proportionally to the diameter of a conductor wire. Further, when the device size reaches a so-called deep submicron, writing to a thermally stable memory by such a magnetic field generated by a current is impossible. This situation is described with reference to FIG. 31.
In FIG. 31, a magnetic field is taken as the ordinate, and a conductor wire diameter is taken as the abscissa. The solid line indicates a magnetic field strength that can be created by flowing a current through a conductor wire on the assumption that a current density satisfies the relationship: i less than 5 MA/cm2. The dotted line indicates a coercive force required when the magnetic storage is thermally stable, and the region on the right side of the dotted line is a thermally stable region and the region on the left side of the dotted line is a thermally unstable region.
The MRAM described above with reference to FIG. 30 has a feature such that writing is conducted using a magnetic field, and has an advantage in that the writing speed is high. However, when the structure of the MRAM is scaled down, the diameter of a conductor wire (for example, a bit line or a word line) used for generating a magnetic field must be reduced. As shown in FIG. 31, the smaller the diameter of a conductor wire, the smaller the strength of the magnetic field generated. That is, writing to a device having a high coercive force is difficult. On the other hand, as also shown in FIG. 31, in the magnetic memory device 131 (see FIG. 30) designed for a small conductor wire diameter, when it has no satisfactorily large coercive force and magnetic anisotropy, it is difficult to keep a magnetic storage against thermal fluctuation. For example, when the conductor wire diameter is about 0.1 xcexcm, a magnetic field at about 50 Oe is required. This means that writing by a current is difficult.
For removing the above-mentioned problem in driving of the magnetization in a scaled-down device by a magnetic field, several proposals have been made.
One example of driving of magnetization by using heat is described with reference to the diagrammatic view of FIG. 32. As shown in FIG. 32, on a substrate 201, a CoFe2O4 layer 211, an Fexe2x80x94Ag layer 212, and a NiFe2O4 layer 213 are stacked on one another, and a circuit 221 for applying a voltage to the Fexe2x80x94Ag layer 212 through the NiFe2O4 layer 213 is provided. The circuit 221 has a construction such that a power source 222 and a switch 223 are connected to each other in series. When a voltage is applied by means of the circuit 221, the exchange coupling between a magnetic vector M in the CoFe2O4 layer 211 in one fixed direction indicated by the arrow shown in the figure and a magnetic vector of the NiFe2O4 layer 213 through the Fexe2x80x94Ag layer 212 is broken, so that the magnetic vector of the NiFe2O4 layer 213 is rendered free. When no voltage is applied from the circuit 221, the magnetic vector of the NiFe2O4 layer 213 is affected so that the direction of its magnetic vector and the direction of the magnetic vector M of the CoFe2O4 layer 211 are the same.
Next, one example of a method for modulating the magnetic anisotropy by using a stress utilizing a magnetostrictive material is described with reference to the diagrammatic view of FIG. 33. As shown in FIG. 33, on an electrode layer 311, a piezoelectric layer 312 and a distortion-sensitive magnetic thin film (which functions also as an electrode) 313 are stacked on one another, and a circuit 321 for applying a voltage to between the distortion-sensitive magnetic thin film 313 and the electrode layer 311 is provided. The circuit 321 has a construction such that a power source 322 and a switch 323 are connected to each other in series. When a voltage is applied by means of the circuit 321, a magnetic vector M indicated by the arrow shown in the figure is generated in the distortion-sensitive magnetic thin film 313.
Next, one example of a method of driving the magnetization by using quantum interference in the multilayer structure is described with reference to the diagrammatic view of FIG. 34. As shown in FIG. 34, on a MgO substrate 401, a Cu under layer 411, a Co magnetic layer 412, a Cu intermediate layer 413, a Co magnetic layer 414, a Cu coating layer 415, a Ge coating layer 416, and a Cu electrode layer 417 are stacked on one another, and a circuit 421 for applying a voltage to between the Cu electrode layer 417 and the Cu coating layer 415 is provided. The circuit 421 has a construction such that a power source 422 and a switch 423 are connected to each other in series.
However, the construction shown in FIG. 32, in which driving of magnetization is conducted using heat, needs a practical method for forming a scaled-down structure which can realize inversion of opposite directions (N/S repeated inversion) of magnetization using only heat.
On the other hand, the construction shown in FIG. 33, in which the magnetic anisotropy is changed using a stress, has problems of integration in the growth of the piezoelectric layer 312 and the removal of fatigue caused by the stress from the material.
Further, the construction shown in FIG. 34, in which driving of magnetization is conducted using quantum interference in the multilayer structure, requires at least two magnetic layers for practical driving of magnetization, and, in this construction, modulation of the magnetization does not always occur only on the surface of the magnetic layer, leading to a necessity that the whole layer structure containing these layers must be formed with high precision.
The present invention provides a method of modulating magnetic properties of materials and a magnetically functioning apparatus for alleviating or solving the above-mentioned problems.
An embodiment of the present invention is a method of changing magnetization state of a magnetic thin film in a stacked multilayer structure comprising following layers: a first layer, a second layer having a higher electrical resistivity than that of the first layer, and a third layer having a lower electrical resistivity than that of the second layer. At least one of the three layers is a magnetic layer having an ordering of microscopic magnetic moments. The method comprises changing the magnetization state of the magnetic thin film by utilizing an effect wherein, when a voltage is applied between the first layer and the third layer by means of an external circuit, electronic state of the surface of the magnetic layer is changed. The change in the electronic state causes modification of either of the following properties: the magnetic anisotropy or magnetostriction of the magnetic layer or the magnetic coupling between the magnetic layer and an adjacent magnetic layer.
The modulation of the above magnetic properties thus obtained leads to deflection or rotation of magnetization vector in the magnetic film, which is utilized in device applications.
Another embodiment of the present invention is a device or apparatus comprising a stacked multi-layer structure comprising a first layer, a second layer having a higher electrical resistivity than that of the first layer, and a third layer having a lower electrical resistivity than that of the second layer. At least one of the above three individual layers is magnetic.
The magnetization vector in this apparatus is deflected or rotated as a consequence of a modulation of either of the following magnetic properties: magnetic anisotropy or magnetostriction of the magnetic layer or the magnetic coupling between the magnetic layer and an adjacent magnetic layer. This modulation of above magnetic properties is achieved by the change in electronic states near the surface or interface of the magnetic layer that is executed by an application of an external voltage to the multi-layer structure.